Scale-up and purification of graphite oxide as intermediate for functionalized graphene

High purity graphite oxide (GO) and scale-up of GO synthesis is in high demand for enabling GO applications in catalysis and the synthesis of functionalized carbon nanomaterials. Although Hummers method, which involves oxidizing graphite with KMnO4 in sulfuric acid, is highly attractive with respect to its cost-effective raw materials and low energy demand, still purification, scale-up and safety problems prompt numerous challenges in large scale production. Conventional dead-end-filtration (DE) is difficult, owing to the severe plugging of the filters by water-swollen GO, whereas dialysis requires massive amounts of water. Plugging problems are solved when using membrane separation with average membrane pores smaller than the lateral micrometer dimensions of GO nanoplatelets. In contrast to DE and dialysis, a short DE prefiltration of acidified GO dispersions, combined with subsequent cross-flow filtration (CF) enables easy GO scale-up and production of high purity GO. Equipped with on-line monitoring of waste-water, this CF process is automated and enables concentration of GO dispersions in the final stage. In GO purification, pH switching represents the key to removing Mn and S impurities.